Bosch Water Heater Error Codes

Common Symptoms

• Err 2 displayed on Bosch Therm — venting or combustion air fault
• Err 10 displayed — no ignition after multiple attempts
• Err 11 displayed — flame lost during operation
• Err 13 displayed — thermistor (temperature sensor) fault
• Err 14 displayed — overheat safety trip or thermal fuse
• Err 15 displayed — heat exchanger overtemperature (scale buildup)
• Err 16 through Err 28 displayed — various temperature sensor or combustion faults
• E1 or E2 on Bosch Tronic or Ariston electric model display

Most Likely Causes

1. 1

   Err 2 — Venting / Combustion Air Issue

   Err 2 indicates the Bosch Therm has detected inadequate flue exhaust or combustion air. The unit requires sufficient fresh air for combustion and an unobstructed path for exhaust gases. Causes: (1) Vent terminal blocked by debris, birds, spider webs, snow, or ice at the exterior wall termination. (2) Exhaust vent pipe length exceeds Bosch's published maximum equivalent length for the unit's BTU rating — excessive vent run creates back-pressure that the draft inducer cannot overcome. (3) Separated or gapped vent pipe joint allowing exhaust recirculation into the combustion air intake (soot staining near a joint is a tell-tale sign). (4) Combustion air intake restricted — if the unit is in a closet or small mechanical room, combustion air must be supplied per code (typically two ventilation openings per Bosch installation requirements). Inspect and clear the vent terminals, inspect the full vent run for joint separation, and verify combustion air supply.

2. 2

   Err 10 — No Ignition (Battery Replacement, Electrode, Gas Supply)

   Err 10 means the Bosch Therm attempted ignition and failed to establish a flame within the trial-for-ignition period. Diagnostic sequence by root cause: (1) AA Battery replacement (Aquastar AQ125BNG, AQ250BNG, and other battery-powered models): the control board and igniter are powered by two AA alkaline batteries. When batteries weaken, ignition spark voltage drops below the level needed to reliably fire the burner. Replace with fresh alkaline AA batteries — this resolves the majority of Err 10 calls on battery-powered models. Do not use lithium AA batteries (voltage profile differs from alkaline). (2) Electrode cleaning: carbon and oxide deposits on the spark electrode prevent reliable ignition spark. Clean the metal electrode tip with 0000 steel wool or fine-grit sandpaper. Verify the electrode gap is 3–4 mm. (3) Gas supply: open the gas shutoff valve fully (handle parallel to pipe). Check that other gas appliances work normally. Measure inlet gas pressure — NG: 4–10.5 in. w.c.; LP: 8–14 in. w.c. On Aquastar LP models (AQ250BNG), verify the LP regulator is correct for the fuel type — LP and NG regulators are not interchangeable. (4) Gas valve fault: if all above are ruled out, the gas valve may be failing to open on demand — requires a licensed technician.

3. 3

   Err 11 — Flame Loss During Operation

   Err 11 means the Bosch Therm established a flame successfully but the flame extinguished during the heating cycle. Causes: (1) Dynamic gas pressure drop — static pressure is adequate but pressure drops below specification under full firing load. Measure dynamic gas pressure during firing at the unit's inlet test port. A pressure drop below 4 in. w.c. (NG) under load indicates an undersized gas supply line or failing regulator. (2) Flame sensor fouling — the flame sensor rod that detects the ionization current of the flame is coated with carbon or oxide deposits, causing it to incorrectly report flame loss. Clean the flame sensor rod with 0000 steel wool. (3) Partial vent blockage — a partial obstruction starves the flame at higher firing rates but allows ignition at low fire, producing Err 11 at higher demand. Inspect the full vent run. (4) Gas valve intermittent fault — the valve opens for ignition but fails to modulate correctly at operating firing rates.

4. 4

   Err 13 — Thermistor Fault (Temperature Sensor)

   Err 13 indicates the Bosch Therm's thermistor (temperature sensor) is reading outside the expected range or has failed open/short. Thermistors are NTC (negative temperature coefficient) resistors — their resistance decreases as temperature increases. A thermistor reading open (infinite resistance) or shorted (near zero ohms) at ambient temperature indicates a failed thermistor. Thermistors can also develop intermittent faults from moisture ingress or wiring connector corrosion. Diagnostic: disconnect the thermistor connector and measure resistance with a multimeter at known ambient temperature — compare to the Bosch resistance-temperature table in the service manual. Replace the thermistor if readings are out of specification. Check the wiring harness connector for corrosion or loose pins before condemning the sensor.

5. 5

   Err 14 — Overheat / Thermal Fuse

   Err 14 means the Bosch Therm's thermal fuse or high-limit overheat sensor has tripped, indicating the unit reached a dangerously high temperature. The thermal fuse is a one-time device that must be replaced after tripping. Root causes of Err 14: (1) Scale buildup causing heat exchanger hot spots — the most common cause on units in hard-water areas. Err 14 typically follows unresolved Err 15 (heat exchanger overtemp) events: scale insulates the heat exchanger, causing progressive overheating until the thermal fuse trips. Always perform a full descale procedure before replacing the thermal fuse — replacing the fuse without descaling causes the new fuse to trip again within one to two heating cycles. (2) Blocked vent or recirculation of exhaust gases (Err 2 conditions that were not addressed) causing combustion chamber overheating. (3) Rare: control board fault causing incorrect gas valve modulation. Order OEM thermal fuse only — aftermarket fuses with incorrect temperature ratings are a safety risk.

6. 6

   Err 15 — Heat Exchanger Overheat (Scale — Descale with CLR)

   Err 15 is the Bosch Therm signature fault for scale-related heat exchanger overtemperature. Calcium carbonate scale deposits inside the heat exchanger passages insulate the copper walls, causing the water passing through to receive insufficient heat transfer and the copper to overheat — the outlet temperature sensor detects a dangerous condition and the unit locks out. Descale procedure using CLR or white vinegar: (1) Shut off gas and electricity. Close cold inlet and hot outlet isolation valves. (2) Connect a small submersible pump to the cold service port with a hose, and run a return hose from the hot service port to a 5-gallon bucket. (3) Fill with 1 gallon of CLR Calcium, Lime & Rust Remover (diluted per CLR label) or 3 gallons of undiluted white vinegar. (4) Open both service valves and circulate for 45–60 minutes. If the solution turns cloudy within 30 minutes, replace with fresh solution. (5) Flush with 5 gallons of clean water. (6) Restore operation. Annual descaling in hard-water areas prevents Err 15 recurrence. Consider installing an upstream phosphate scale inhibitor cartridge for ongoing protection.

7. 7

   Err 16 — Outlet Overtemperature

   Err 16 means the hot water outlet temperature sensor has detected a temperature above the safe limit (typically above 120°C / 248°F). Causes: (1) Temperature setpoint set too high — lower the setpoint to 120°F (49°C) maximum for standard residential use. (2) Low flow rate while the unit is firing at full BTU output — insufficient water flow causes the small water volume in the heat exchanger to heat to dangerously high temperatures. Check for flow restrictions (clogged aerators, partially closed valves, flow below 0.5 GPM minimum). (3) Recirculation loop running with no demand — if a recirculation pump is running while all hot fixtures are closed, water circulates through the unit with no heat removal, building temperature. Verify recirculation pump scheduling.

8. 8

   Err 17 — Burner Abnormality

   Err 17 indicates the Bosch Therm's combustion monitoring system has detected abnormal burner behavior — typically a flame that ignites and appears stable but with abnormal characteristics detected by the flame sensor ionization reading. Causes: (1) Gas supply pressure outside specification — both too low and too high gas pressure produce abnormal combustion. Check inlet gas pressure (NG: 4–10.5 in. w.c.; LP: 8–14 in. w.c.). On LP models, an overfilled or underperforming LP regulator is a common cause. (2) Burner scale or debris — fine debris or scale particles on the burner ports produce irregular combustion. Inspect and clean the burner ports. (3) Gas valve modulation fault — the proportional gas valve is not delivering correct fuel quantity for the firing demand. Requires technician diagnosis.

9. 9

   Err 20 / Err 21 / Err 23 / Err 24 / Err 28 — Temperature Sensor Codes

   Bosch Therm uses multiple thermistors and temperature sensors to monitor the heating process. Each sensor has a dedicated error code: Err 20 = DHW (domestic hot water) outlet sensor fault. Err 21 = inlet (cold water) sensor fault. Err 23 = heat exchanger sensor fault. Err 24 = exhaust temperature sensor fault. Err 28 = bypass valve sensor fault. These sensor codes typically indicate: (1) A sensor that has failed open or short — disconnect the sensor and test resistance against Bosch's temperature-resistance spec. (2) A wiring harness connector that has developed corrosion or a loose pin — clean the connector contacts with electrical contact cleaner and reseat. (3) Actual temperature at the sensor is outside the expected range (e.g., Err 24 exhaust sensor may indicate a genuine exhaust overtemperature from a vent blockage — inspect the vent before replacing the sensor). In cold climates, Err 21 (inlet sensor) can appear transiently in winter if the cold supply temperature drops below the sensor's expected minimum reading — this is typically a self-clearing fault.

10. 10

    Bosch Tronic E1 — Upper Thermostat Fault (Reset Button, Element Test)

    E1 on Bosch Tronic (electric point-of-use) and Ariston (storage) models indicates the upper thermostat has tripped or failed. The reset button is located on the upper thermostat body, accessible behind the upper access panel (typically 2–4 screws). Press the red or black reset button firmly until it clicks. Restore power and test. If E1 clears and does not return: the thermostat tripped from a one-time overheat — no replacement needed. If E1 returns within a heating cycle: test the upper element resistance with a multimeter (disconnect leads, measure across element terminals) — a healthy Bosch Tronic element reads 12–16 Ω. Open (OL) or near-zero resistance means element replacement is needed. Also verify the 120V or 240V supply voltage is correct for the unit's nameplate rating.

11. 11

    Bosch Tronic E2 — Lower Thermostat Fault (Reset Button, Element Test)

    E2 on Bosch Tronic and Ariston models indicates the lower thermostat has tripped or failed. The lower reset button is on the lower thermostat body, accessible behind the lower access panel. Press firmly until it clicks. If E2 clears and does not return, the thermostat tripped once from overheat — often caused by sediment insulating the lower element on Ariston storage tanks. Flush the tank (drain and refill until water runs clear) before testing the element. If E2 returns after reset: test the lower element resistance (12–16 Ω expected). On Ariston storage tanks, heavy sediment on the lower element is the most common cause of repeated E2 trips — flush the tank thoroughly and replace the lower element if resistance is out of spec.

Quick DIY Checks

1. 1Step 1 — Read and note the error code: for Bosch Therm tankless, the Err code is displayed on the digital panel on the front of the unit. Write down the exact code. For Bosch Tronic or Ariston electric, note the E1 or E2 displayed on the LED indicator strip. Do not clear the code before documenting it — the code identifies the fault category and guides all subsequent steps. If the unit has no display and no error code but won't produce hot water, verify that the unit has power (Tronic: check GFCI and circuit breaker; Therm: check batteries and wall outlet) before assuming a fault code is hidden.
2. 2Step 2 — Try the reset procedure first: for Bosch Therm: press the on/off button to turn the unit off, wait 30 seconds, and press on. Alternatively, unplug the unit from the wall outlet (or remove batteries on battery-powered Aquastar), wait 60 seconds, and restore power. Trigger a hot water draw and observe whether the Err code returns. A code that clears and does not return for 24 hours or more may have been a transient condition (power fluctuation, brief gas interruption, momentary sensor spike). A code that returns within one or two heating cycles indicates an active fault requiring diagnosis. For Bosch Tronic E1/E2: reset is done via the physical reset button on the thermostat — see cause descriptions above.
3. 3Step 3 — Err 10 on battery-powered models: replace AA batteries immediately: if the unit is an Aquastar AQ125BNG, AQ250BNG, or any battery-powered Bosch Therm, battery replacement is the first action — not the last resort. Open the battery compartment (front of unit or dedicated panel). Remove old batteries. Install two fresh alkaline AA batteries (or D-cell per your model). Restore the unit and test a hot draw. If Err 10 clears and the unit fires normally: batteries were the cause. Cost: under $5. If Err 10 persists after fresh batteries: proceed to electrode cleaning and gas supply check.

1. 4Step 4 — Err 15: descale the heat exchanger with CLR or vinegar: this is a DIY-approachable procedure. Turn off gas and electricity. Close cold inlet and hot outlet isolation valves at the unit service ports. Connect a small submersible pump (120 GPH minimum) to the cold service port with a 3/8-inch hose; run a return hose from the hot service port to a 5-gallon bucket. Add 1 gallon of CLR diluted per CLR instructions, or 3 gallons of undiluted white distilled vinegar. Open both service valves and start the pump. Circulate for 45–60 minutes — the solution will turn milky as dissolved scale is carried out. If it saturates in under 30 minutes, replace with fresh solution. After descaling: flush with 5 gallons of clean water. Restore operation. Err 15 should not reappear. Schedule annual descaling in hard-water areas.
2. 5Step 5 — Err 2: inspect and clear vent obstructions: go outside and visually inspect the vent termination cap and combustion air intake opening. Check for bird nests, spider webs, ice, leaves, or debris blocking the openings. Clear any obstructions. Return inside and walk the vent pipe run looking for separated joints, gaps, or soot deposits near joints (indicating exhaust recirculation). Reconnect any separated joints and seal with foil HVAC tape. Verify the vent run length — consult the Bosch installation manual for your unit's maximum equivalent vent length. If the run exceeds the limit, a vent run reduction or booster fan is needed. Reset the unit after clearing the vent.
3. 6Step 6 — Err 14: descale before replacing the thermal fuse: the thermal fuse is a one-time device that must be physically replaced — resetting the unit will not restore a tripped thermal fuse. But before replacing the fuse, complete the full Err 15 descale procedure (Step 4 above). Replacing the fuse on a scaled heat exchanger causes the new fuse to trip again within one to two heating cycles. After descaling: order an OEM Bosch thermal fuse for your specific Therm model. Locate the thermal fuse on the heat exchanger assembly (position varies by model — consult the Bosch service manual). Relieve pressure, remove the old fuse, install the OEM replacement, and restore operation.
4. 7Step 7 — Sensor codes (Err 13, 20, 21, 23, 24, 28): check wiring connectors before replacing sensors: unplug the affected sensor's wiring connector and inspect the pins for green corrosion, bent pins, or debris. Clean with electrical contact cleaner and reseat firmly. Reconnect and reset the unit. If the Err code clears, connector corrosion was the cause. If it returns: use a multimeter on resistance mode to test the thermistor at known ambient temperature — compare to the resistance-temperature curve in the Bosch service manual. If out of specification, replace the sensor. Sensor replacement on Bosch Therm is typically a 30–60 minute job — the sensor is a single connector and one or two mounting screws.

Repair vs Replace

Recommended Tools & Parts

• AA Alkaline Batteries (4-pack)

  Fresh alkaline AA batteries for Aquastar AQ125BNG, AQ250BNG, and other battery-powered Bosch Therm models. First step for any Err 10 on battery-powered models.

  $4–$8

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• CLR Calcium Lime & Rust Remover (28 oz)

  For descaling the Bosch Therm heat exchanger to resolve Err 15 and prevent Err 14. Dilute per CLR instructions. Circulate for 45 minutes max, then flush thoroughly.

  $8–$15

  Buy on Amazon →

• Bosch Therm Thermal Fuse (OEM)

  OEM replacement one-time thermal fuse for Bosch Therm models — required after Err 14 trip. Always descale the heat exchanger before installing. Must match your model's temperature rating.

  $20–$55

  Buy on Amazon →

• Bosch Therm Thermistor / Temperature Sensor

  Replacement thermistor for Err 13, Err 20, Err 21, Err 23, Err 24, or Err 28. Check connector condition first before replacing. Must match your Therm model number.

  $20–$50

  Buy on Amazon →

• Submersible Pump + Hose Kit

  For descaling the Bosch Therm heat exchanger (Err 15) — circulates CLR or vinegar through the service port connections.

  $25–$50

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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